Paper coating refers to its coating with various substances, the mostly used being pigment-containing coating colours. Other generally used coatings include waxes, plastics, silicone, surface sizes and starch. The purpose of coating is to fill the irregularities on the paper surface with one or more coating layers. Coating primarily affects the printability and appearance of paper, but also the stiffness of paper and its resistance to water, grease or solvents.
A coating is composed of an aqueous solution of one or more pigments, one or more binders, and additives. For some special papers, solvent-based coatings are used. Accordingly, pigments, binders and additives constitute the components of a coating. A coating is prepared at a coating kitchen by mixing these components. The mixing can be performed either in doses as batch production or by continuous mixing. The proportions or amounts of the different components are given in colour formulas. From the coating kitchen, the coating is transferred to a coating head, where it is evenly applied onto the surface of a paper web. The thickness, i.e. amount (g/m2) of the coating layer is adjusted suitable by peeling excess coating off the paper by means of a doctor blade. The amount of the coating is adjusted by changing the position of the doctor blade relative to the paper web. Excess coating is directed to a circulation, from where it is reapplied onto the surface of the paper. The coating layer remaining on the surface of the paper is dried by evaporating the excess water contained by the coating with coating drying units located after the coating head. Either both or only one side of the paper can be coated by using either separate coating heads or by coating both sides simultaneously. The coatings on the different sides of the paper may also be identical or different depending on the unequal sidedness of the base or the operational asymmetry required by the end product.
For adjustment of the amount of a coating on the paper, the amount of the coating remaining on the paper is measured and, based on said measurement, the position of the doctor blade is changed relative to the paper web, if required, to change the amount of the coating. Nowadays, the amount of a paper coating is measured by IR measurement. U.S. publication 4,957,770 discloses a method based on IR measurement for sensing the amount of a coating on a moving base. Measuring the amount of a coating by IR measurement is based on a measurement of the ratio of the reflection intensities of the IR absorption wavelength bands and reference wavelength bands characteristic of the different components of the coating and water and fibres. The ratio of the reflection intensities of the absorption wavelength bands and the reference wavelength bands corresponding to the components enables the determination of the amount of each component in the coating remaining in the paper. The total amount of coating remaining in the paper can be determined based on the amount of a component, once the formula of the coating applied onto the paper, i.e. the substances mixed into the coating and the amounts thereof are known.
However, a problem in present solutions for measuring the amount of a coating is that the composition of the coating has to correspond to the formula exactly in order for the amount of the coating remaining in the paper to be determined correctly. Because of the recycling of coating or problems associated with its manufacture, the composition of the coating may change, i.e. the coating no longer corresponds to the formula. When the coating does not correspond to the formula, the total amount of the coating in the paper cannot be determined exactly. In batch processes, the composition of the coating may differ from what is stated in the formula for instance because of inaccuracy in metering the components of the coating, transition points occurring in grade changes or incomplete washing of coating treatment devices. In a continuous coating production process, inaccuracy is caused for instance by inaccuracy in metering measurements, unfamiliarity of recycled pigments precipitated in washing and grade changes.